EVALUATING THE CYLINDER HEAD

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

CLEANING THE CYLINDER HEAD
VISUALLY INSPECTING THE CYLINDER HEAD
CHECKING CYLINDER HEAD FLATNESS

CLEANING THE CYLINDER HEAD

The cylinder head seals the upper end of the cylinder. A gasket between the cylinder head and engine block helps to create an airtight seal in the combustion chamber; it is this seal that permits compression of the fuel air mixture on the piston compression stroke. Since the cylinder head must effectively seal the cylinder for proper engine operation, it must be free of cracks, warping, and other damage.

Carbon buildup and gasket residue: when a cylinder head is removed for an engine overhaul, it should be thoroughly cleaned with solvent before any evaluations of its condition are made. The combustion chamber area of the head may require special attention. Carbon buildup from combustion can form on the head surface. In fact, carbon buildup can materialize on any surface which is exposed to the burning fuel air mixture. Try using a wooden scraper, putty knife, or wire brush to remove this stubborn residue.

There may also be gasket remnants left on the cylinder head and engine block after they have been separated. Such leftover gasket material can be removed in the same way as carbon buildup.

Take care not to dig into the cylinder head metal with your scraper or brush as you are cleaning it. This is particularly germane to modern aluminum head motors; aluminum is a soft metal that incurs damage quite easily. After the carbon residue has been scraped off, the cylinder head can be cleaned again in solvent. Be sure to clean between its cooling fins, where the head is subject to dirt and debris accumulation that can reduce its ability to cool.

The spark plug hole: when the head is removed, you can easily clean and check the condition of the spark plug hole threads. The threads in an aluminum cylinder head are relatively easy to damage, and therefore should always be checked. If necessary, threads can be repaired either by running a thread tap through the spark plug hole, or installing a new threaded insert. Generally speaking, you should run a tap through the threads even if they appear to be in good condition; this will remove any carbon buildup in the threads.

VISUALLY INSPECTING THE CYLINDER HEAD

When the head has been thoroughly cleaned, you can check it for signs of damage. Look for minute cracks, and other kinds of damage in the combustion chamber area especially. If any cooling fins are broken, the head should be replaced; even one broken or corrupted fin can heavily impact engine temperatures. The heads in old L head motors are highly reliable, and therefore seldom damaged. The more exotic heads of OHV and OHC powerplants are slightly more prone to damage, but should withstand the rigors of normal engine operation. The most common cylinder head problem is thread damage in the spark plug hole.

CHECKING CYLINDER HEAD FLATNESS

If the cylinder head appears to be free of damage, you can then check the flatness of its gasket surface. A cylinder head gasket can compensate for slight variations in flatness, but, for the most part, the head needs to be flat or the seal between it and the engine block will fail. An engine service manual will provide the maximum amount of surface warping permissible in a reusable cylinder head. In most engines, the amount of warping allowed is no more than 0.003 inches. There are two methods that can be used to check head flatness; both use feeler gauge blades to measure the actual warping.

The straightedge method: one way to check for cylinder head flatness is with a straightedge such as a metal ruler. The head should be positioned so that its gasket surface faces up. Then the straightedge is placed across the head surface. If you notice clearance between the straightedge and head surface, you will insert feeler gauge blades until one is found that reflects the warpage at that point; this will be the largest feeler gauge blade that can be fitted into the space between straightedge and head.

Since the straightedge is narrow, several locations on a cylinder head should be checked; also be sure to measure head flatness in several different directions across the head surface. For an accurate flatness measurement, the single best posture for the straightedge is diagonally across the surface of the head. Some engine manufacturers will specify particular points at which these flatness measurements ought to be taken. Regardless where a measurement is taken, it should be compared to the service manual specification for maximum allowable cylinder head warpage.

The surface plate method: you can also use a surface plate to check cylinder head flatness. A surface plate, in this instance, is a flat piece of material with a smooth surface. Special surface plates are available specifically for warpage measurements, however they provide greater precision than necessary for cylinder head evaluation. A good surface plate for checking a cylinder head can be a simple pane of glass(a windowpane for instance). Glass not only has a perfectly flat surface, it is inexpensive and readily available.

When you have the surface plate ready, place the cylinder head on it with the gasket surface facing down. Then use feeler gauge blades to measure any clearance you find between the surface plate and cylinder head. Feel for gaps around the entire head circumference. Compare the largest clearance found to the service manual specification for maximum allowable cylinder head warpage. If you discover that head flatness is within these specifications, the head can be reinstalled on the engine. If, however, there is greater warp than allowed, the head must be either resurfaced by a machine shop or replaced. In most instances, cylinder heads on small engines are small and reasonably inexpensive, making it more cost effective to replace one. On larger engines, and on certain OHV and OHC engines, a new head may be expensive enough to make resurfacing the existing one a better option. Before making that decision, analyze the costs of resurfacing versus replacement.